Two major sources of amplifier error are non-linearity and thermal distortion. The mechanisms which produce these undesirable traits are inherent in the fundamental physical properties of semiconductor pn junctions. The signal-amplitude error in an uncompensated wide-band amplifier may be as high as ten percent; however, the incorporation into the circuit of complex compensating networks permits sophisticated measurement instruments to have a rated amplifier precision in the one- to three-precent range.
There are many applications in which amplifiers having a high degree of precision, e.g., 0.01 to 0.1 percent, are required, particularly in measurement instruments. One well-known method of reducing amplifier error is through the use of feedback techniques. In feedback amplifiers, the final output is sensed and fed back to the input so that linearity errors and thermal distortion are cancelled to a large degree. While feedback amplifiers having very high precision may be designed, such high-precision feedback amplifiers are intended for only very low frequency operation because they have several limitations which make them unsuitable for high-precision wide-band signal processing. For example, adequate damping becomes difficult to obtain as the frequency increases. Also, small inherent time delays around the feedback loop to the input cause the output to be out of phase with the input. Thus, phase distortion is introduced and such phase distortion increases as the frequency increases, degrading amplifier precision accordingly. While some operational amplifiers are capable of wide-band operation to perhaps several hundred megahertz, DC operational amplifiers having precision in the 0.001 to 0.01 percent range have a maximum effective bandwidth from DC to only 20 or 50 kilohertz since precision degrades rapidly at the higher frequencies.